class CancelException implements ExecuteException {
  final String message;

  CancelException(this.message);

  @override
  String toString() {
    if (message == null) return "CancelException";
    return "CancelException: $message";
  }
}

class ExecuteException implements Exception {
  final String message;

  ExecuteException(this.message);

  @override
  String toString() {
    if (message == null) return "ExecuteException";
    return "ExecuteException: $message";
  }
}
